M., I assume you mean a CT scan? My kids haven't needed them but my siblings did when they were younger and I have some friends who have children with chronic health issues/disabilities who need periodic CT scans as part of their regular health care and none have suffered any obvious issues from their scans. Your carefully weigh the benefits and risks. If the risk of not having the scan are greater than the risks that could potentially come down the line years later due to radiation exposure, you have the scan done. It's important to be aware of the risks of repeated exposure so that scans aren't done when they aren't really necessary, but sometimes they are a critical diagnostic tool. It's really hard to find a clear cause and effect anyway...there are studies that indicate a correlation in cancer rates and increase exposure to radiation through a variety of exposure types, so it's an important thing to be aware of, but you never know which risks one is exposed to over a lifetime actually trigger a disease like cancer.
For what it's worth, I had to have a CT scan while pregnant with twins to rule out a suspected blood clot. The risk of dying from a blood clot was greater than the risk of exposing the babies to radiation in utero, so it was a prudent choice. The kids are fine.
Rather than focus on one test your child had, focus on her overall health and well being and do what you can to boost her immune system, set her up for a lifetime of healthy habits, limit her exposure to toxins in the air, food, and environment, etc. There is no point in worrying yourself over this.